Because basic properties of stimuli such as similarity-dissimilarity and novelty-familiarity are intrinsically relational and are embedded in ubiquitous environmental contexts, the ability to detect such relational information is essential to adaptive functioning. There is substantial evidence that children that have low mental ages-both retarded children and young non-retarded children--exhibit markedly deficient performances on tasks that demand relational responding (Greenfield, 1985; Soraci et al., 1987, 1989). In an extension of experimental paradigms the present investigator has utilized in previous research, the specific aim of this proposal is to analyze and enhance the relational characteristics of stimulus arrays in order to facilitate performance on tasks such as oddity and match-to"sample. Oddity and arbitrary match-to-sample are particularly clear examples of tasks in which individual stimuli do not have discrete informational value independent of their relations to the stimuli that comprise their context; rather, the informational value of individual stimuli in these tasks is in large part determined by the surround in which they appear (Dinsmoor, 1985; Sidman & Stoddard, 1968). Converging evidence from various studies conducted by the present investigator and others suggests that a critical factor in the performance discrepancies between retarded and non-retarded children is a differential sensitivity to relational information (e.g., Soraci, Deckner, Baumeister, 1989). A major goal of the proposed research is to provide evidence relevant to the development of a theoretical model of relational learning. In the proposed studies, emphasis is placed on the role of the structure of the visual arrays in facilitating detection of relevant stimulus relations. Stimulus organization variables are also examined as they relate to stimulus detection and discriminative responding. The proposed studies are divided into two sections: Oddity and Stimulus Organization.